Conformational Plasticity of the MAGE-A3 Protein as a Therapeutic Strategy in Multiple Myeloma

Abstract

Type I MAGE proteins interact with the RING domain protein Kap1 through their conserved MAGE Homology Domains (MHD) to form E3 ubiquitin (Ub) ligases, which ubiquitinylate p53 targeting it for proteasomal degradation. RNAi experiments demonstrated that MAGE-A3 inhibits p53-dependent and independent mechanisms of apoptosis and confers resistance to chemotherapy-induced apoptosis in human myeloma cell lines. Since MAGE expression correlates with progression of multiple myeloma (MM), preventing the interaction with Kap1 is a promising therapeutic intervention against MM. The MHD of MAGE proteins are made of two winged helix domains (WH) linked by a flexible β-hairpin. Structures of MAGE proteins suggest that the WH domains need to undergo a conformational change from a closed to an open form to interact with Kap1 and activate its Ub-ligase activity. Virtual screening on the closed form of MAGE-A3 identified two compounds that recapitulate the RNAi experiments, suggesting that they may inhibit the interaction of MAGE-A3 with Kap1 leading to the apoptosis of MM cells. Further experiments are being conducted to elucidate the nature of the observed effect. MD simulations of the complexes of MAGE-A3 with the small molecules show binding modes in the groove between the two WH domains. These may be responsible for inhibiting the conformational transition. Further simulations to estimate the effect of the small molecules on the conformational change as well as refinement of the initial leads to improve the affinity and selectivity of the compounds are under way. Supported by NIH R21 CA191898.